this is actually a workaround for a bug in gcc, whereby it asserts
inequality of the keys being compared...

note that this header is still bogus and needs a lot of work and
factoring into arch-dependent parts...

It's not necessary to save any registers on the stack across syscall in
x86_64 __set_thread_area.  Don't waste cycles or bytes on it.

this was originally written for an early draft of the library where
non-standard functions would reside in a static library separate from
the shared libc.so, which would implement a pure standard. the idea
was not to depend on an implementation-dependent __syscall_ret
function in the main libc. but it turned out to be better to put
everything in a single library for both static and dynamic linking
uses, and thus the (incomplete) remnants of this feature were just
enlarging the source and binary.

to i386.